1. Field of the Invention
The present invention relates to frequency compounding, and more particularly, to an apparatus and method of frequency compounding to perform contrast imaging using ultrasound to enhance the sensitivity to contrast agents having different sized microbubbles.
2. Description of the Related Art
The use of ultrasound imaging has grown quickly due to the image quality achievable, its safety, and its low cost. Such imaging can be broken into two general categories: tissue imaging and contrast imaging. In contrast imaging, contrast agents, for example microbubbles of heavy gas encapsulated in rupturable shells of material, are introduced intravenously into the bloodstream. Due to their physical characteristics, contrast agents stand out in ultrasound examinations and therefore can be used as markers that identify the amount of blood flowing to or through the observed tissue. In particular, the contrast agents resonate in the presence of ultrasonic fields producing radial oscillations that can be easily detected and imaged. Normally, this response is imaged at the second harmonic of the transmit frequency fo.
Recently, it has been determined that tissue also produces harmonic responses which influence the images produced during contrast imaging. Several techniques have been developed which take advantage of the primarily linear response behavior of tissue to cancel or attenuate the linear tissue signals. In several of these techniques, multiple transmit lines are fired along the same line of sight into the body. The transmit waveform is modified (e.g., in terms of power, phase, or polarity) from line to line to produce a variation in the response received by the transducer. These data points are then processed to remove the influence of their linear components to yield data that primarily contains the non-linear response of the contrast agents.
Although the above-described techniques work well in removing the influence of stationary tissue, there are some more interesting non-linear effects that occur. For example, what makes a microbubble that is, e.g., four microns in diameter, able to be seen with ultrasound is that it resonates and when the ultrasound hits the microbubble, the microbubble expands and contracts and actually rings like a bell and emits a strong enough ultrasound signal back so as to be detected. Most of the contrast agents that are being developed by drug companies presently have microbubbles of widely varying diameters. Their sizes are not well controlled. Some of the drug companies are able to control the diameter very well, but most of them are not, and typically the range of microbubble sizes would be from below one micron up to 20 microns (xcexcm) or more. The bigger microbubbles, bigger than seven microns (xcexcm) in an intravenous injection, will get filtered out by the lungs. They will get stuck in the lungs and then metabolize so that there remains a range of microbubbles from seven to sub micron size. The size of a microbubble corresponds to the frequency at which it resonates.
Smaller microbubbles resonate at higher frequencies and bigger microbubbles resonate at lower frequencies. Microbubbles have a fairly high Q, which means that they are fairly selective at how much they resonate with respect to their natural frequency. If a certain microbubble that wants to resonate at one frequency is hit with an ultrasound with another frequency, the microbubble will not resonate nearly as much as it does when hit with its favorite frequency, more or less. Thus, if imaging is performed using a certain frequency, it is possible to really only be able to excite maximally the population of microbubbles which have the size that corresponds to that frequency.
From the above, it can be appreciated that it would be desirable to develop an apparatus and method of contrast imaging in which the response to microbubbles of varying sizes is maximized so as to enhance the imaging sensitivity of the contrast agents.
The present invention relates to an apparatus and method of imaging contrast agents within a patient""s body. The apparatus produces an image of a body part into which a contrast agent has been introduced. The apparatus comprises an imaging system transmitting at multiple frequencies to elicit a response from the contrast agent at corresponding multiple frequencies, forms images corresponding to each frequency, and then combines the formed images.
The method comprises transmitting transmit signals at multiple frequencies to elicit a response from the contrast agent at corresponding multiple frequencies, receiving the transmit signals reflected off the contrast agent, forming images corresponding to each frequency, and combining the formed images.